I was just curious on why you see people catch saltwater fish and then as soon as they get the fish to the boat they grab the leader and then cut the line? I didn't know if this was a regulation or something? Thanks for the help y'all!

When fishing in saltwater you do not always have control of what bites the hook.  I personally do not want to keep a shark or bring a 7 footer on board a 21' boot, that's a lot to handle.  I don't even want to unhook one like this.

http://www.bassresource.com/bass-fishing-forums/gallery/image/1453-shark-003/

^^^ Correctamundo

There are also some fish that you are not legally to remove from the water in some places.

All the above. I believe you are not allowed to remove grouper from the water in FL. So you either cut the line if you can't get the hook out of the mouth in a safe manner to the fish. Could be wrong, but I believe I saw this on Saltwater Experience on the formerly Versus channel, now NBC Sports...

As Roger Maltbie says, "You want play this game ya gotta know the rules".  Florida recognizes many federal restrictions on fish, can't speak for other states.

There is a long list of sharks that are prohibited to keep.  So many sharks do look alike to the non expert, not a bad idea to release them all unless you are 100% positive, it isn't worth a fine.  Grouper are fair game when of legal size and in season, Goliath grouper aka Jewfish and Nassau grouper are prohibited.  There quite a few other species of fish and invertebrates that are prohibited as well, too long a list to jot them all down.